Figures suggesting that fake anti-virus packages are allowing cybercrooks to make more than €10m a month are been described as little better than guesswork. Vendors across the industry are warning that scarewore packages - which attempt to trick would-be marks into handing over their hard-earned cash for packages that claim to resolve fictitious infections - are a growing problem.

But estimates by Panda Security that 30 million Windows PCs have been infected with fake antivirus programs have met with skepticism from a rival vendor. Graham Cluley, senior technology consultant at Sophos, question Panda's assumption in reaching its figures, which he suggests are little better than back of the envelope calculations.

Panda has tracked almost 7,000 variants of the type of malware over the last year alone. Prospective marks are often served up this type of malicious content after browsing pornographic sites, responding to fake electronic greeting cards or downloading warez. In one case documented by Panda, a fake Google toolbar was used to promote the scam.

These various tricks are all designed to redirect users onto web pages selling fake antivirus products, designed to imitate the look and feel of legitimate packages (as illustrated by screen shots by Panda here: http://www.flickr.com/photos/panda_security/tags/fakeantivirus ). These programs follow a common pattern: users are warned that they are infected with malware via pop-up windows, desktops, and screensavers that keep appearing and getting in the way of using a computer normally. The goal is to scare the less tech savvy into shelling out for products that are worse than useless.

None of these packages detect and defend against malware and some go as far as installing adware or, in extreme cases, banking Trojans on compromised PCs.

Dominic Hoskins, country manager Panda Security UK, explained: "The information we have at present suggests that some 3 per cent of these users have provided their personal details in the process of buying a product that claims to disinfect their computers. In fact, they never even receive the product. Extrapolating from an average European price of €49.95, we can calculate that the creators of these programs are receiving more than €10 million per month".

Other security watchers criticise the assumptions Panda makes in arriving at its figures.

Graham Cluley, senior technology consultant at Sophos, told El Reg: "It looks like Panda says they looked at two million computers, and found three per cent were infected with fake anti-virus. In other words, 60,000 computers."

"They've then taken Forrester's estimate of how many computers there will be by the end of 2008 and extrapolated that there must - therefore - be 30 million computers infected by fake anti-virus. That's quite a jump, and I think a flawed one."

"If the 30 million infections is true, how do they know it's not the same people being infected over-and-over again? How do they know three per cent of people end up paying for the bogus security software?"

Panda's general warning and valid and timely even though its figures are open to question. Questions about the financial impact of malware naturally arise but pining down reliable figures is a notorious tricky business, a problem we've seen before - and will undoubtedly see again.